1. Industrial Field of the Invention
The present invention relates to a semiconductor device comprising a thin film transistor (TFT) on an insulating substrate such as of glass, and it also relates to a process for fabricating the same.
2. Prior Art
Semiconductor devices comprising TFTs on an insulating substrate (such as a glass substrate) known heretofore include an active matrix-addressed liquid crystal display device using the TFTs for driving the matrices and an image sensor.
The TFTs utilized in those devices generally employ a thin film silicon semiconductor. Thin film semiconductors can be roughly classified into two; one is a type comprising amorphous silicon semiconductor (a-Si), and the other is a type comprising crystalline silicon semiconductors. Amorphous silicon semiconductors are most prevailing, because they can be fabricated relatively easily by a vapor phase process at a low temperature, and because they can be readily obtained by mass production. The physical properties thereof, such as electric conductivity, however, are still inferior as compared with those of a crystalline silicon semiconductor. Thus, to implement devices operating at even higher speed, it has been keenly demanded to establish a process for fabricating TFTs comprising crystalline silicon semiconductors. Known crystalline semiconductors include polycrystalline silicon, microcrystalline silicon, amorphous silicon partly comprising crystalline components, and semiamorphous silicon which exhibits an intermediate state between crystalline silicon and amorphous silicon.
A thin film of a crystalline silicon semiconductor enumerated above can be fabricated by any of the following known processes:
(1) A process which comprises directly depositing a crystalline film in the step of film deposition; PA0 (2) A process which comprises depositing an amorphous semiconductor film, and then irradiating a laser beam to the film to obtain a crystallized semiconductor by taking advantage of the laser beam energy; and PA0 (3) A process which comprises depositing an amorphous semiconductor film, and then applying thermal energy to crystallize the film to obtain a crystalline semiconductor.
With respect to the first process above, it is technologically unfeasible to form a uniform film having favorable semiconductor properties over the entire surface of the substrate. Moreover, this process is uneconomical, because it excludes the use of a low cost glass substrate due to the presence of a film deposition step which requires a temperature as high as 600.degree. C. or even higher.
The second process can be exemplified by a process employing the most commonly used laser at present, i.e., an excimer laser. Considering the too small area of a laser beam can irradiate at a time, this process is still disadvantageous in that it can only afford a low throughput. Furthermore, the laser is not sufficiently stable to cover the entire surface of a large area substrate with a uniform film. Thus, it can be safely said that this process awaits a forthcoming technology.
The third process is superior to the first and the second processes above concerning its applicability to the formation of large area films. However, it also requires a high temperature of 600.degree. C. or higher during the film deposition. This process again excludes the use of inexpensive glass substrates. Thus, it is required to further lower the heating temperature during the film formation.
In particular, even larger displays are required in the present day liquid crystal display devices. Accordingly, those devices keenly demand their implementation using larger glass substrates. A heating treatment is indispensable for the fabrication of semiconductors, however, shrinking and deformation occur on a glass substrate during the thermal treatment. These dimensional change in glass substrate considerably impair the precision at, for example, the mask alignment. Such an instability in dimensional precision has been found as a great problem in the process for fabricating semiconductors. The most widely used 7059 glass (a product of Corning Corp.) undergoes deformation at a temperature of 593.degree. C. Accordingly, it cannot resist to the conventional thermal crystallization treatment without being deformed. Moreover, the step of thermal crystallization in the known processes consumes such a long time amounting to several tens of hours, or even longer. It is therefore keenly demanded to develop a rapid step for the crystallization.